This invention relates generally to a system and method for reducing pressure in a container pressurized with a fluid provided to the container from a pressurizing fluid source. In a particular aspect, the invention relates to a system and method for testing a test fluid under pressure in a laboratory, wherein the pressure can be decreased in small increments.
In the oil and gas industry cement slurries are pumped into well bores for various reasons, one of which is for cementing casing. Different well bores can have different temperature and pressure conditions which can affect different cement slurries; therefore, it is desirable to be able to test a particular cement slurry for its suitability for a particular well bore environment. Such testing is well known in the industry.
A typical cement test occurs in a closed container to which heat can be applied to heat a sample of the cement to a desired temperature (e.g., 27.degree.-205.degree. C.) . The contents of the closed container are also maintained under pressure (e.g., 500-20,000 psi). The temperature and pressure are typically those anticipated to be encountered downhole. This type of testing device includes autoclaves. Specific examples of such test equipment are the Halliburton Services Cement Consistometer and the Halliburton Services Ultra-sonic Cement Analyzer.
In such high pressure devices, there has been a need for a pressure limiting mechanism to limit pressure increases such as result from temperature increases brought about by the heating which is typically performed at the beginning of a test. This pressure limiting has been done by a technician monitoring a pressure gauge and manually opening a valve to relieve pressure. A preferable way would be to use a valve which would automatically release a limited volume of pressurized fluid from the test container to produce a drop in pressure when the pressure reaches a predetermined level. This would require a controllable relief valve, but relief valves which are rated at 20,000 pounds per square inch (psi), which is a typical pressure rating on cement testing equipment of the type mentioned hereinabove, are not common, and those presently available typically relieve a much larger volume than is acceptable to maintain sufficient pressure to continue the desired cement test in the example of the Halliburton Services Ultra-sonic Cement Analyzer.
One valve which has a suitable pressure rating is available from Autoclave Engineers. In a test using this valve, it was determined that the valve reaction time was not acceptable and that too much pressure was relieved from the same cement testing container such as is used in the aforementioned Ultra-sonic Cement Analyzer. We also believe that the large mass of the piston and inherent friction of the packing seal of the Autoclave Engineers valve would be detrimental to the accurate control needed with the particular type of cement testing equipment referred to hereinabove.
Another valve that has been proposed for controlling pressure is disclosed in U.S. Pat. No. 4,917,349 to Surjaatmadja et al. Although this valve has proved useful in some applications, it also has a response that can be too slow for small volume, high pressure applications. That is, when it is opened it releases too much pressure before it can be re-closed even though closure can occur within less than a second. This not only results in erroneous pressure control, but also the cement or other test fluid can be expelled into attached plumbing due to the rapid depressurization.
Because such valves as referred to above are alone not fast enough to release sufficiently small volumes of high pressure fluid to properly control the pressure in small volume, high pressure containers such as are used in the Ultra-sonic Cement Analyzer, there is the need for an improved system and method by which such control can be obtained.